Hollow cathode floating zone melter and process



www wmf-mmm SARCH ROUE Sept. 24, 1968 l. DRANGEI. ETAL 3,403,007

HOLLOW CATHODE FLOATING ZNE MELTER AND PROCESS Filed April 20, 1966 2 Sheets-Sheet 1 Sept. 24, 1968 DRANGEL ET AL 3,403,007

HOIJLOW CATHODE FLOATING ZONE MELTER AND PROCESS 2 Sheets-Sheet Filed April 20, 1966 s s M R d 0 Av. p mm? o @M m60@ A @mw Ei b9 'digg/A 3,403,007 HOLLOW CATHODE FLOATING ZONE MELTER AND PROCESS Irwin Drangel, Yonkers, Patrick J. McMahon, New

Rochelle, and Walter Class, Yonkers, N.Y., assignors to Materials Research Corporation, Orangeburg, N.Y., a corporation of New York Filed Apr. 20, 1966, Ser. No. 543,830 14 Claims. (Cl. 23-301) ABSTRACT OF THE DISCLOSURE Zone refining a body of crystalline material is carried out by positioning said body in a plasma region of a hollow cathode discharge tube having an anode independent of the material, and a plurality of spaced cathodes defining a hollow space whereupon an impressed high Ipotential establishes a high temperature plasma producing a molten zone in said material which zone is moved in an axial direction along said material.

This invention relates to apparatus and process for handling fusible materials by floating zone melting of such materials either to purify them by zone refinement or to grow single crystals, and more particularly to the provision of a new heat source Vfor establishment of the' molten-zone which is particularly well suited for use with high melting point inorganic, non-metallic materials.

Zone melting, a form of ifractional crystallization, is the term used to describe the movement of impurities or solutesin crystalline materials. This process is usually accomplished by slowly passing a short molten zone through a comparatively long solid charge of material, the advancing edge of the zone being the melting interface, and the trailing edge the freezing interface. The moltenizone is able to move or distribute any impurities (solutes) mainly by virtue of the conditions at the freezing interface. The basic factors involved are the difference between the solubility of the impurity diffusion in the solid material compared to its speed of diffusion or move-a ment away from the freezing interface into the body of the liquid zone.

This process and apparatus are available not only for purification but for growing of single crystals.

Zone melting or zone refining per se is known as is the so-called floating zone process in which the material to be treated, in rod form is disposed in vertical position, its molten zone being held in place by its surface tension. Such a floating zone process is, for example, dis closed in Theuerer U.S. Patent No. 3,060,123 of Oct. 23, 1962 and elsewhere in the literature, as, for example, P. H. Kech and I. I. E. Golay 1953, 89 Physical Review 1297 and Pfann U.S. Patent No. 2,739,088 lof Mar. 20, 1956. These techniques have allowed the process of floating zone refining to be carried out without the disadvantage of contamination of the -fusible material in the molten zone by materials `from a container.

Many different types of heat sources have been employed for establishing the molten zone. Induction heat ing, resistance wire-wound furnaces, radiant energy heat lamps, and a focused beam of electrons have been most frequently employed. The electron beam heretofore has been the most efiicient heating source and as such is particularly desirable for high melting lpoint materials. It has one disadvantage, namely, that the high vacuum required for its use causes the decomposition of most compound materials.

Objects and .features of the invention are the provision of novel apparatus and process -for providing a superior 3,403,007 Patented Sept. 24, 1968 heat source overcoming the disadvantages of electron beam heating noted as well as being more efficient and simpler than any of the other earlier known types of providing heat for establishing the molten zone.

In accomplishing these objects and features, a new hollow cathode technique has been devised that circumvents the heating problems of the known arrangements.

This new technique will be referred to as the hollow cathode method. It enables operation at pressures of -1 mm. Hg as compared with the electron beam process which requires pressures below 10-4 mm. Hg. Further more, this new system can frequently be operated using a gas which is one of the decomposition species of the material to be processed, thereby suppressing the decom-1 position by the law of mass action. A description of the hollow cathode technique in its present state of de velopment and a discussion of its potential will be given below.

When the cathode of a self-sustaining gas discharge is formed into a cavity, it is found that under certain con-1 ditions, depending upon gas species, gas pressure, cathode geometry, cathode material and discharge current, the discharge will change from the normal plane discharge to a different mode. The onset of this mode is characterized bya drop in the applied voltage and/or a large increase in discharge current. It is visually observable as a discharge of intense luminescence located within the cavity offfthe cathode. This effect is known as the hollow cathode effect. Our experiments have shown that a material placed within the highly luminescent plasma of such a discharge is subject to intense heating.

Other objects and features of the invention will be comev apparent from the following specification and the accompanying drawings, wherein:

FIGURE 1 is a diagrammatic sectional view of a dis-s charge tube and characteristics of a plane self-sustaining glow discharge;

FIGURE 2 is a similar view illustrating characteristics utilizing principles of this inventon and embodying a hollow cathode; and

lFIGURE 3 is a diagrammatic elevation of a practical embodiment for practicing this invention.

Referring to the drawing and first to FIGURE l, the proposed heating technique of this invention may best be understood if one considers the processes responsible for the establishment of a self-sustaining plane cathode dis charge. A schematic discharge tube 10 along with its pertinent characteristics is shown in FIGURE 1. The maior part of such a tube 10 is filled with the soicalled positive column 11, which is a luminescent region of vrelatively high conductivity which has no function in the maintenance of the discharge other than that of providing a conducting path to the anode 12. This is evidenced by the fact that the anode 12 may be moved through this entire region without any large change in the voltagecurrent characteristics of the discharge tube 10. The posi tive column 11 is separated from another luminescent region 13, called the negative glow region, by the Faraday dark space 14. Between the negative glow region 13 and the cathode 15 is another dark region 16, called the cathode dark space or the cathode fall region.

Electrically, a gas discharge is different from an ordi nary DC resistance in that the voltage drop across the discharge does not vary linearly with the dimensions of the discharge. Instead, the largest voltage drop occurs across the cathode fall region 16 which is also a region of high positive ion density (FIG. 1). The results of many different investigations have shown rather conclusively that electrons emitted from the cathode 15 are accelerated by the strong electric field in the cathode dark space 16 to generate ions in the cathode glow region 13. These ions in turn are accelerated towards the cathode where they cause the electron emission from the said cathode, which thereby maintains the discharge.

Now consider a discharge tube (FIG. 2) utilizing a hollow cathode 21 and an anode 22. The voltage drops and the ion densities are as shown in the latter figure when the hollow cathode discharge mode is occurring. Under these conditions, electrons emitted from one 4side of the cathode 21 will accelerate toward the other side, whereupon they are scattered back toward their origin. Similarly, ions produced in the cathode glow region 23 are scattered back and forth due to the influence of the positive space charge in the cathode dark space 24. The hollow cathode 21 therefore acts as a trap for both the electrons and the ions which oscillate across the cavity. The luminescent region 23 inside the hollow is a neutral region containing equal numbers of positive and negative space charges and is thus by definition a plasma. No unique temperature can be attributed to the plasma because the ion and gas molecule temperatures are different from the electron temperatures which are quite high due to the energy gained in the cathode fall region 24.

It is believed that the heating of a body placed in the luminescent region or plasma 23 results from the bom bardment of the body by the high energy electrons. This heating is, therefore, related to the cathode fall voltage which is in turn very nearly equal to the external voltage applied to the discharge. Melting in the plasma is, there-1 fore, achieved when a critical energy density of bombarding electrons is achieved. This energy density in ergs/ crn.2 is given by the equation E=EeXJ7 where Ee is the average electron energy in ergs and I is the flux of electrons in cm.-2 impinging on the surface. From this expression, it is evident that the necessary energy density can be achieved -by a large flux of relatively low energy electrons, or by a relatively small flux of high energy electrons. The formation of a molten zone is acn complished more easily in the small flux case because under these conditions the space charge dispersion of the electrons in the plasma 23 is much smaller. Consequently, for a fixed hollow geometry, the molten zone formed will be more localized. and will have greater mechanical stability. High voltage, low current operation of the plasma generator is, therefore, to be desired. This can be accomplished by selecting a cathode material which exhibits a high cathode drop in the discharge gas. Furthermore, the shape of the hollow cathode also has a very significant effect upon the voltage level. The optimization of the hollow cathode plasma process, therefore, involves a selection of cathode material as well as cathode geometry. The advantages of the hollow cathode process over the electron beam process result from the higher pressures at which the hollow cathode process will permit heating by electron bombardment. The importance of high pressure operation in the appropriate gas may be understood in the case of an oxide if one again considers the reaction which leads to decomposition:

M203 ZMOT -I- TO (2 The equilibrium pressure of MO is related to the oxygen pressure in the system by the equilibrium constant. Conse quently, processing of the oxide in any oxygen plasma will help suppress the decomposition by the well-known law of mass action.

The rate of decomposition is also dependent upon the rate at which the `decomposition species diffuses from the region of the molten zone. An estimate of the influence which the system pressure will have upon this kinetic phenomenon can be obtained from an ideal gas law prediction of the gaseous diffusion coefficient, D in cm2/sec., which gives;

L5 D (s) where E=the average molecular velocity in cm./sec. and may be further expressed as:

where:

K=Boltmanns constant,

T :the absolute temperature, in degrees K.),

P=the pressure, in dynes/cm-2,

d and m=the molecular cross-section in cm., and mass,

in grams, respectively,

Combining Equations 3, 4 and 5 then gives the expression:

2 spitz ma (s) From the pressure dependence of Equation 6, it immediately follows that the `diffusion constant is l04 times greater in the electron beam environment as compared with the hollow cathode environment. Thus, in the hol= low cathode environment, the rate of decomposition species effusion from the molten zone is suppressed by the law of mass action, and the rate of diffusion is suppressed by virtue of the pressure sensitivity of the gaseous diffusion coefficient.

A further decomposition suppression mechanism arises by virtue of the ion trapping which occurs in the electrostatic `elds of the hollow cathode. This probably leads to a large random current of oscillating ions in the plasma 23. This current does not result in a net charge transfer and is, therefore, not measured yas a discharge current. However, the random current does cause a large ion impingement upon the surface of the molten zone. This impingement rate is statistically equivalent to a partial pressure of oxygen ions which are chemically equivalent to oxygen atoms. It may, therefore, be predicted that the oxygen pressure inside the hollow cathode is probably larger than the chamber pressure by virtue of this phenomenon.

A practical embodiment of novel apparatus utilizing the principles discussed relative to FIGURE 2 above which has been designed, manufactured and utilized by us is shown schematically in FIGURE 3. In this embodiment, a body 36 of aluminum oxide A1203 or titanium dioxide TiO2 or other high melting point oxide in rod or bar form prepared in any suitable manner as by casting or cutting is supported at bot-h ends 31 and 32, for example, with chucks 33 and 34 in a chamber 35. The body 30 is preferably of the highest purity attainable by prior known processes and `all elements in its environment are of materials of thehighest purity. The wall 36 of the chamber 35 may be pure silica, while the chucks 33 and 34 may be of spectroscopically pure graphite bored and tapped for graphite clamping screws 36. The ends of chamber 35 are closed as by metal heads 37 and 38 sealed to the silica wall 36 as by lead gaskets 39, 40.

A zone 41 in the bar 30 is melted by the movable hollow cathode arrangement 42 of this invention. This hollow cathode ararngement 42 includes a -generally cylindrical cathode carrier frame or jacket 43 of pure ceramic insulation material, one of whose end closures 44 has a large central opening 4S. This end closure is provided with a metallic shield 46 on its external surface. The shield 46 has a central opening 47 of smaller dimensions than the opening 45, but of substantially larger diameter than rod 30, so as not to contact the latter during traverse of the arrangement 42 along the rod 30'. The other end closure 48 is provided with a centrally located, inwardly projecting tubular shaft portion y49 whose inner diameter likewise `substantially exceeds the diameter of rod 30 to avoid contact therewith during traverse of arrangement 42 along said rod. A ring-like metallic cathode holder 50 preferably of `pure aluminum or nickel is located internally of the 'cylindrical ceramic cathode carrier fra-me 43. This metallic holder 50 is provided with internal cooling passages 51 for transmit therethrough of cooling fluid such as water which is delivered thereto to opposite ends of said passages 51 via inlet and outlet ducts 53 and 54 provided.in the ceramic carrier 43 which ducts in turn are connected by exible metallic hoses 55 and 56 of nickel or aluminum to inlet and outlet nipples 57 and 58 provided, for example, in the end wall 38 with appropriate sealing and insulating gaskets 59 and 60. Cooling medium, such as water from a source may be circulated through the cooling passages 51 in holder 50 from said source via nipples 57 and 58 and hoses 55 and 56.

The upper part of the central hole 61 of metallic holder 50, in the embodiment shown, Ihas approximately the same diameter as opening 45 in the end 44of the ceramic in- Isulation jacket 43. This hole 61 serves to receive a pair of cathode inserts 62 and 63 of nickel or aluminum. Obviously, since these cathodes are of metal in direct contact with the cooled metallic holder, they, too, will be cooled. These inserts have outer skirt portions 62a and 63a which t respectively into recesses in holder 50 surrounding the tubular shaft portion .49-which, together with 'said holder, serve to mount and tix the cathode inserts 62 and 63 in holder 50. These inserts have semiellipsoidal facing cavities -64 and 65 defining the hollow cathode cavity or region 66, which in the embodiment shown is an ellipsoid with 1A and 3A minor and major axes. Aligned circular openings 67 and 68 in the upper and lower walls of the inserts defining the ellipsoidal space 66 are dimensioned to prevent physical contact between the rod 30 and said inserts 62 and'63, during traversing irnotion of the cathode arrangement 42 and also to permit access of gas supplied to the said hollow cathode region 66. This gas may be supplied via an entry duct 69 to the chamber 35 provided in cover 37 and exhausted via exhaust duct 70 in the bottom cover38.

A metallic anode 71 is positioned within chamber 35 preferably on the bottom cover 38 and is connected via a terminal 72 and lead 73 to the positive terminal of a high volta-ge source (not shown). The negative terminal of this voltage source is connected by a lead 73 to an insulated terminal 74 mounted in the ,lower end cover 38 of chamber 35 and by a flexible lead 75 inside of chamber 3S to the conductive cathode carrier ring 50, thus providing high potential of the order of 1.5 to 2 kv. between the'anode inserts 62 and 63 and cathode 71 inside of chamber 35 and a particular concentration of heat plasma (corresponding to plasma 23 of FIG. 2) in the molten zone 41 which as depicted lies in the ellipsoidal cathode region 66.

The entire cathode arrangement 42f-is movable in the lon-gitudinal direction of the rod 31 within chamber 35 as by a drive mechanism including .a shaft 76 secured at its inner end to the bottom wall 48 of the ceramic insulation jacket 43 and passing downwardly and outwardly through a seal 77 in the lower chamber head 37 which permits its axial movement. In its outer portion shaft 76 has a rack 78 which meshes with a driving -gear 79 that may be appropriately driven in either direction to thereby cause relatively slow movement of the cathode arrange ment along the length of rod 30 within chamber 35 from a lowermost position in proximity to chuck 34 to an uppermost position in proximity to chuck 33 and return, or vice versa..

In operation, assuming the cathode arrangement 42 in a position ladjacent either to chuck 33 or 34, a reduced gas pressure of from 0.2 to l mm. of mercury is effected within chamber 35 by delivery and exhaustion of a gas such as oxygen to said chamber 35 via inlets and outlets 69 and 70 and established and maintained. The electric power source is connected to provide a potential of 1.5 to 2 klv. between the anode 71 and hollow cathode inserts 62 and 63,

thus developing the intensely hot plasma within the ellip-l soidal hollow cathode region 66, through which rod 30 extends, thereby providing the limited molten zone 41 in said rod. The current at said voltages ranges from 350 to 1500 ma. Invthe meantime, cooling fluid is circulated through the cathode holder 50. Obviously, the cathodes 62 and 63 held by the holder are likewise cooled.

After establishment of the molten zone 41 in rod 30,5 i adjacent either the chuck '33 or chuck 34, the driving" y slowly moving `cathode plasma cool after having been` purified in the molten zone 41 leave a purified rod residue or a single crystal behind the moving molten zone 41.

In the embodiment shown, the rods range in diameter from 1/8 to 3/16 inch and are of aluminum tri-oxide A1203 or titanium dioxide TiO2 or other high melting point oxides.

The hollow cathode technique of this invention can also be utilized for the growth of single crystals of a desired Orientation with the apparatus of FIGURE 3. A single crystal may be grown by employing a seed having the orientation kdesired as a growing matrix, mounting it in the chuck 34, suspending a rod from the chuck 33 with its free end in contact with the free end of the crystal seed, establishing the molten zone 41 at the seedrod interface so that it extends from the rod to the seed and moving the molten zone upwardly slowly along the 1. The process of successively zone-melting and re` solidifying an elongated body of fusible material which comprises sippprting the body for traverse along its lengftfh;Z of a hollowj'space defined yby at least a pair of cooled* cathodes ifafconfined enclosure maintained at sub-atmospheric gas pressure, applying a high potential between the cooled cathodes and an anode distinct from the body and independent thereof, to create a high temperature plasma about the body in said hollow space that creates a molten zone extending through the entire cross section of the body within said hollow space and displacing the cathode, the plasma and molten zone in axial direction along said body.

2. The process according to claim 1, including maintaining the enclosure of the cathodes and anode and body to define a confined space with the anode remote from the cathodes and independent of the body and also main taining said sub-atmospheric pressure at from 0.2 t0 1 mm. of Hg in said confined space.

3. The process of claim 2, in which said gas pressure is provided by oxygen and said body is of an oxide of a metal in Group III of the periodic table of elements.

4. The process according to claim 1, including the step of cooling the cathodes by circulation of cooling medium therearound during maintenance of said high potential.

5. The process according to claim 1, wherein said cathodes have hollow shapes to provide ellipsoidal configuration to said hollow space within which said plasma is created -by said high potential.

6. Apparatus for successively zone-melting and resolidfying a body of fusible material comprising a chamber, means within sa-id chamber for supporting said body therein, a hollow cathode arrangement including cathodes defining a hollow space surrounding said body and movable in an axial direction along the latter, means for so moving said hollow cathode arrangement, an anode within said chamber independent of said body, means for cooling said cathode arrangement, means for supplying a high potential between said cathode arrangement and said anode and thereby establishing a high temperature plasma within said hollow space that acts on said body to create a molten zone extending through the entire cross-section of said -body within said hollow space and which zone is movable along said body as said hollow cathode arrangement is similarly moved.

7, Apparatus according to claim 6, including means for introducing and maintaining a gaseous atmosphere within said chamber at subatmospheric pressure of :from 0.2 to l mm. of Hg.

8. Apparatus according to claim 6i, wherein said hollow cathode arrangement includes a metallic carrier for said cathode, and said cooling means includes means for circulating a cooling medium through said carrier.

9. Apparatus according to claim 6, wherein said cathodes have elliptical facing cavities defining said hollow space which has overall ellipsoidal shape through which said body extends.

10. Apparatus according to claim 6, wherein said hollow cathode arrangement includes a metallic carrier for said cathodes, an insulative jacket for said carrier and wherein said cooling means for said carrier has passages for circulation therethrough of cooling medium, and means for supplying said cooling medium thereto.v

11. Apparatus according to claim 10, wherein said means for supplying said cooling medium includes flexible tubing connected to said passages.

12. Apparatus according to claim 6, wherein said hollow cathode arrangement comprises a metallic carrier into which said cathodes are mounted and supported in spaced relationship relative to said body and wherein said'cathodes are separate inserts having complemental facing cavities defining said hollow space which has a selected overall shape, an insulating jacket surrounding said carrier and a metallic shield on a portion of said jacket.

13. Apparatus according to claim 6, wherein said means for supporting said body comprise a pair of chucks engagea-ble respectively with opposite ends of said body.

14. Apparatus according to claim 6, wherein said means for moving said hollow cathode arrangement axially along said body includes a rod extending outwardly of said chamber and means externally of the latter for imparting driving motion thereto in reciprocal directions.

References Cited UNITED STATES PATENTS 2,809,905 10./ 1957 Davis. 3,136,915 6/1964 Jaatineu. 3,210,518 10/1965 Morley. 3,250,842 5/1966 Hikido.

NORMAN YUDKOFF, Primary Examiner.

G. P. HINES, .Assistant Examiner. 

